1. Field of the Invention
The present invention is related to the field of geophysical exploration. More specifically, the present invention is related to a system for acquiring and recording seismic signals measured by a sensor.
2. Discussion of the Relevant Art
Equipment which is used for geophysical surveying typically includes a digital data recorder. The digital data recorder generates digital samples, at spaced apart time intervals, of signals generated by at least one seismic sensor responsive to a seismic energy source. The digital data recorder typically can also store the digital samples or transmit the samples to another device capable of storing the digital samples.
In a typical seismic survey, the seismic energy source is actuated, and the signals generated by the at least one sensor are conducted to the digital data recorder.
The digital samples which are made and stored in the digital data recorder during the seismic survey represent the amplitude of the signals at the instant in time at which each of the samples is made. Processing of the digital signal samples is generally performed after they are stored in the recorder. Processing is performed in order to determine certain characteristics of an earth formation, such as acoustic velocity and the possible presence of acoustic impedance discontinuities in the earth formation.
Various methods are known in the art for generating digital samples and storing the signal samples in formats which facilitate the processing. For example, one method of generating digital samples includes the use of a floating point analog-to-digital converter (ADC). The floating point ADC measures the amplitude of the signal at spaced apart time intervals, and converts each measurement of the amplitude into a multiple-bit binary number. The floating point ADC begins sampling only when controlled to do so by an external command signal, such as an activation signal sent to the seismic energy source to actuate the source. Floating point ADC's therefore are easy to synchronize with the actuation of the source, so that the time at which any particular signal sample is acquired relative to the source actuation can be accurately determined.
A limitation of the floating point ADC is that it requires, interposed between it and the sensor, an analog anti-alias filter having specific response characteristics. The analog anti-alias filter removes components of the signal which have a frequency above a maximum sampling frequency. The maximum sampling frequency typically is one-half the frequency (the inverse of the elapsed time between the spaced apart time intervals) at which the ADC operates. In order to preserve information contained in the signal at frequencies near the maximum sampling frequency, the analog anti-alias filter must have a so-called "sharp roll-off" characteristic, that is, the filter must decrease response rapidly with increasing frequency above a maximum passband frequency of the filter. Analog anti-alias filters having sharp roll-off are difficult and expensive to build.
It is also known in the art to generate digital samples of an analog signal, such as the signal from the seismic sensor, by using a delta-sigma converter. The delta-sigma converter comprises a single-bit digitizer, called a modulator, and an integrator. The output of the modulator comprises a serial stream of very short duration digital bits which are non-zero for an equivalent duration of time proportional to the amplitude of the signal being digitized. The modulator typically operates at a frequency much higher than the maximum frequency contained in the signal. For example, seismic survey signals of interest typically have components extending only to a frequency of 2,000 Hz, but a modulator used in seismic survey equipment can operate at 1,024,000 Hz (1.024 mHz). Because the modulator operates at a very high frequency relative to the frequency of the signals being sampled, it is typically not necessary to include a sharp roll-off anti-alias filter in seismic survey equipment using delta-sigma converters.
The output of the delta- sigma modulator is typically conducted to the integrator, which provides a digital signal output comprising multiple-bit binary numbers generated at a much lower frequency than the operating frequency of the modulator. A typical integrator used in seismic survey equipment operates at a frequency of 4,000 Hz. Because the digital samples from the modulator are used to generate an output from the integrator at a much lower frequency than the operating frequency of the modulator, it is necessary to filter high-frequency components out of the samples conducted from the integrator to avoid aliasing. The components which must be filtered out of the modulator output typically have a frequency of one-half or more of the operating frequency of the integrator. For example, in the integrator used in seismic survey equipment as previously discussed, components having a frequency above 2,000 Hz should be filtered out of the modulator output. Resampling anti-alias filtering is typically performed by a digital filter, such as a finite impulse response (FIR) filter.
A drawback to using delta-sigma converters is that the delta-sigma converter is free running, meaning that it does not begin sampling under control of an external command signal. The delta-sigma converter is therefore difficult to synchronize to an external time reference, which for example can be the previously described activation signal used to actuate the seismic energy source.
It is known in the art to use digital finite impulse response (FIR) filters, similar to those used in anti-alias filtering the output of the delta-signal modulator, in order to synchronize the output of a delta-sigma converter to a predetermined time reference. Digital signal samples which are output from the integrator are processed through the FIR filter so that a reconstructed digital signal sample value can be calculated exactly at a time value which is coincident with the predetermined time reference. Subsequent digital signal samples spaced apart in time at intervals corresponding to the output frequency of the delta-sigma modulator, or to any other sample rate lower than the integrator operating frequency, can also be calculated by using the FIR filter.
A drawback to seismic survey equipment systems known in the art which use delta-sigma converters and FIR filters is that they typically require use of a two-stage or multiple-stage FIR filter. The samples output from the converter must be FIR filtered once to remove frequency components in the modulator output above the maximum frequency capable of being processed in the integrator, and FIR filtered a second time to synchronize the once-filtered samples with the predetermined time reference. Using two-stage or multiple-stage digital FIR filters can increase distortion of components of the signal which have frequencies within the filter passbands, because each stage of the digital FIR filter introduces a so-called "passband ripple" to the signal response in the passband, the distortion compounding with each filter stage. Passband ripple is an artifact of the mathematical algorithm of any digital filter, and cannot be completely eliminated from any individual digital filter stage.
Another drawback to using two-stage or multiple-stage FIR filters is that it is often impossible to generate digital signal samples containing higher seismic frequencies, typically above 1,000 Hz, using a two or more stage FIR filter because this filter can use an excessive amount of computation time. Typically, the two-stage FIR filter used in seismic survey equipment is limited to use with signal frequencies of about 125 Hz, which corresponds to a digital sample rate of 2 milliseconds, or a maximum sampling frequency of about 500 Hz.
Digital data recorders known in the art also include circuits to remove low level direct current (DC) voltages. DC may be impressed on analog signal lines from seismic sensors by various sources such as biasing errors in analog signal amplifiers forming part of a seismic recording system, or by rectification of signals generated by radio equipment or electric power lines which can occur in analog amplifier circuits in the recording system. It is preferable to remove the DC voltages from the signal before recording the digitized signal so that signals from a plurality of sensors recorded simultaneously will all be recorded as having substantially the same output level when there is no seismic energy input to the sensors. It is known in the art to remove the DC voltages by interposing an analog high-pass filter between the sensor and the digital data recorder. A limitation of using a high-pass filter is that it is very difficult to design an analog filter which can provide the required DC removal without also attenuating low frequency seismic signals in the range of 0.1 to 10 Hz. Attenuation of low frequency seismic signals can reduce the quality of information obtained from the seismic survey.
It is an object of the present invention to provide a digital recording system for seismic surveys which employs a delta-sigma converter using a single-stage FIR filter to perform the processes of bandpass filtering, resampling and synchronization.
It is a further object of the present invention to provide a means for adaptive DC removal for a seismic recording system without using a high-pass filter.